A cantilever array sensor has attracted wide interest by having various advantages such as real-time and label-free detection characteristics (Non-Patent Document 1). Applications including an electric nose (Non-Patent Documents 2 and 3) and chemical and biological detection (Non-Patent Documents 4 to 9) that employ a cantilever array have been proposed. Most of this research employs optical reading using a laser reflected from a surface of a cantilever, which causes several important problems with respect to the actual application of this technique. First of all, a laser device and its peripheral devices for reading are expensive and are difficult to miniaturize. In addition, target molecules in an opaque liquid such as blood can not be detected by the optical reading technique because a signal is considerably attenuated and a refractive index is considerably changed, which is not suitable for use.
One of the most promising solutions for these problems is a piezoresistive cantilever array sensor (Non-Patent Document 10). In a cantilever sensor, a sample is adsorbed onto a receptor layer fabricated in advance on a surface of a cantilever to induce surface stress, which deflects the cantilever. Thus, by detecting the deflection, it is possible to detect the sample. FIG. 1 illustrates an example of a structure of a piezoresistive cantilever array sensor, which is a cross-sectional view of a part close to a fixed end of a cantilever. In this example, a piezoresistive member is built into a surface of the cantilever and is protected by a nitride film. Thus, the upward/downward deflection of the cantilever due to stress on the surface of the cantilever causes compression/elongation strain in the piezoresistive member, which changes the resistance of the piezoresistive member. Such a change of the piezoresistive member caused by the surface stress is detected by an electric circuit as schematically shown in FIG. 2. As shown in the figure, four sides, that is, the piezoresistive member of the measured cantilever shown in FIG. 1, a reference cantilever and two resistors form a bridge. Here, a change in resistance of one side, that is, the piezoresistive member in the measured cantilever can be detected, in a state where voltage is applied to a pair of corners of the bridge, on the basis of voltage of the other pair of opposing corners.
As understood from the above-mentioned operating principle, the piezoresistive cantilever array sensor does not need complex and bulky peripheral devices relating to optical reading. Further, the piezoresistive cantilever array sensor can be manufactured in the same process as a complementary metal-oxide semiconductor (CMOS), and is thus possible to be low cost due to mass production and integrated into existing semiconductor devices such as a mobile phone due to micro miniaturization. Further, this sensor is usable for detection in an arbitrary opaque liquid. Although the piezoresistive cantilever array sensor has these attractive advantages, this type of sensor is not yet sufficiently optimized.
In order to enhance sensitivity of the piezoresistive cantilever, various solutions have been proposed so far. Non-Patent Document 11 discloses a technique in which various factors such as art annealing time, a doping level and a measuring frequency are considered to obtain a signal-to-noise (S/N) ratio of about 10. It is shown that a multilayer cantilever using a residual stress in each layer has superior curvature and sensitivity (Non-Patent Document 12). Various shapes such as various positions of a patterned surface or a receptor layer are discussed to obtain an improvement of sensitivity of several tens of percent (Non-Patent Document 13). However, all this research is based on a normal cantilever shape having a free end, and has a basic problem of the piezoresistive cantilever, that is, a problem that sensitivity is low with respect to stress of the entire surface caused by a sample uniformly distributed without concentration.
A solution for optimizing the piezoresistive cantilever toward a detection application, that is, detection of surface stress caused by a sample is not the same as a solution for a normal atomic force microscope (AFM) or an optical reading cantilever. A sensor for the AFM is based on “a point force”, that is, a force applied to a probe disposed at a free end of a scanning cantilever. On the other hand, a cantilever sensor is based on “surface stress” uniformly induced on an overall surface of a cantilever (Non-Patent Documents 14 and 15). Respective portions of the cantilever are equivalently deflected by the surface stress, and as a result, displacement is accumulated toward the free end. Thus, the displacement is at a maximum at the free end. In the optical reading system, a laser is typically reflected at a free end of a cantilever. Thus, the entire surface stress induced on the cantilever can be efficiently detected.
On the other hand, a signal of the piezoresistive cantilever does not depend on the displacement of the free end, but depends on the stress induced in a piezoresistor. In the case of the scanning cantilever, where the point force is applied to the free end, the stress is concentrated in the vicinity of a fixed end, but in the case of the cantilever sensor, only a part of the stress induced by the sample can be detected by the piezoresistor. It is because this stress uniformly spreads on the entire surface. Thus, in order to acquire larger stress in the piezoresistive part to achieve higher sensitivity, another optimal solution for the piezoresistive cantilever array sensor is necessary.
Non Patent Document 16 discloses a vertically layered cantilever structure for improvement in sensitivity. However, in this design, a double-layer structure is necessary, which causes difficulties in manufacturing.